In machine tool spindle support systems, it has been the practice to utilize antifriction type bearings such as roller and ball bearings for supporting a tool spindle in its housing. Some support systems require extreme accuracy and stiffness not found in roller type bearing elements, which can introduce cyclical irregularities into a support system. In grinding machines in particular, it has been found that the hydrostatic bearing system (that is, where high pressure pockets of oil are located at various peripheral points around a bearing diameter of a spindle), is utilized to provide high stiffness and good centrality of location. The hydrostatic bearing system tends to be self-restoring to the central state as a spindle is deflected. Several disadvantages are inherent in a hydrostatic bearing system, however, namely extremely close tolerances must be provided across the leakage land of the bearing to permit the escape of fluid and consequent pressure drop due to the leakage flow. Secondly, extremely high pressures, i.e. in the nature of 500 lbs. per square inch or more must be provided by a pump system to the hydrostatic pockets.
Spindle support system have attempted to utilize another hydraulic type of journal bearing: the hydrodynamic bearing. In the hydrodynamic bearing, a plurality of tilting pad shoes are employed around the pheriphery of a spindle bearing diameter, and low pressure oil is permitted to flood the shoe bearings and spindle diameter. When the spindle is rotated at the running rpm. (generally a high rpm., in the nature of 1000 rpm. or higher), the spindle exerts a frictional drag on the fluid bath and drives a wedge of oil into the clearance between the spindle and the tilting bearing pad. The advantage of the hydrodynamic bearing is that extremely high stiffness of a bearing system may be generated by the rotating spindle, but the stiffness at starting up speeds and slow down speeds is extremely low, since the pressure is low in the bearing clearance area. Consequently, during start up and slow down conditions, the spindle will actually rub on the lowermost shoe of a multishoe support system. In time, wear can occur on both the shoes and the spindle bearing diameters, thus greatly effecting spindle bearing performance and life.
The life of the spindle bearings is thus directly related to the number of start-ups and shut-downs of a spindle bearing system. Of course, the spindle start-up condition would exist at least once a day when starting a machine for a work shift, and as wheel changes are required, the spindle must be stopped as well.
Applicant has obviated the difficulties inherent in the prior art hydrodynamic bearing systems, by a novel bearing system which employs a hydrostatic bearing integral with the hydrodynamic system, and the hydrostatic bearing is automatically shut off as the hydrodynamic bearing is run at the operating condition.